The present invention relates to a stencil printing machine for conducting printing on a printing material by using a stencil sheet.
In a stencil printing method, ink such as emulsion ink is provided to one surface of a stencil sheet with a perforated image formed therein, a printing sheet is placed on the other surface of the stencil sheet, and then the ink is pressed to transfer through the perforated image onto the printing sheet.
As a method for exerting pressure on the ink to transfer, there has been mentioned a flat press method and a squeegee method. In the flat method, whole area of the stencil sheet is uniformly pressed at one operation. In the squeegee method, pressure is exerted in a linear form on the stencil sheet while shifting by moving a squeegee plate or a squeegee roller along the stencil sheet.
In both of the methods described above, the pressurized ink portions press other ink portions below. That is, the ink itself functions as a pressure transmitting substance for extruding the ink. As to the ink used in the methods described above, as the fluidity or softness of ink is increased, it permeates faster into the printing sheet. This decreases a drawback caused by set-off, i.e. ink stain on the back surfaces of the printed sheets stacked. In this situation, however, capillarity phenomenon arises between the stencil sheet and the printing sheet, thereby causing an excessive-ink flow and ink bleeding to deteriorate quality of the printed images.
Conversely, ink with low fluidity and softness, i.e. hard ink, does not cause capillarity phenomenon easily, thereby improving quality of the printed images. In this situation, however, there arise other problems such that it takes a long time to permit ink to permeate into the printing sheet after ink-transfer on the printing sheet, and that set-off occurs since excess ink is transferred to the printing sheet because of high viscosity thereof.
Further, in the flat press method, although there is an advantage such that it uses a simple mechanism, more solid ink is used owing to a fear of ink-leakage which worsens the drawback of set-off.
On the other hand, the squeegee method is complex and large-scale since it requires high squeegee pressure.
Accordingly, an object of the present invention is to provide a stencil printing method and a stencil printing machine which overcome the contradictory phenomena previously explained, and can provide stencil printing with an excellent printing quality and less set-off.